You're right about how ETFs work, but that's not how futures or options contracts work. Since the futures will be settled in cash then at no point will actual BTC be involved at all.

A good analogy is if we make a bet. You bet me $100 that Bitcoin will be $20,000+ by the end of 2018. Then if it is you get $100 and if it isn't I get $100. At no point was any actual BTC bought or sold as part of our bet.

This is how I understand these futures contracts will work, so unless I went and somehow otherwise affected the BTC market to keep the price under $20k so i could win the bet the price of BTC shouldn't be affected by our bet at all.